If the NPC after saying hello has a limited pie menu of social actions, or if you have to make them selectable to even be able to interact with them, chances are they're the "bad" kind of NPC. Basically, if they only have special menu options, or if you can't interact with them at all, you shouldn't try to do anything with them. A few examples are Crumplebotton, Grim, taxi/bus/carpool drivers, or similar.